The present invention relates to methods and apparatus for generating light such as ultraviolet light from excimer-forming gases.
There has been a need for improved light sources capable of generating ultraviolet light in the spectral region of between about 50 and 200 nanometers wavelength, commonly referred to as the xe2x80x9cvacuum ultravioletxe2x80x9d or xe2x80x9cVUVxe2x80x9d region. VUV photons have energies on the order of 10 electron volts (10 eV) and are capable of breaking chemical bonds of many compounds. Thus VUV light can be used to accelerate chemical reactions as in chemical vapor deposition, curing of photosensitive material, production of ozone, and cracking gaseous waste products. Moreover, the minimum feature size that can be imaged with light is directly proportional with the wavelength of the light. VUV light has the shortest wavelength of any light that can be focused and reflected with conventional optical elements. Therefore, photographic processes employing VUV lights can image smaller feature sizes than those imaged with other light wavelengths. This is of particular importance in photographic processes used to fabricate semiconductors. In addition, such microimaging of features requires high brightness of light sources with such short wavelengths.
Additional needs exist for broadband VUv light sources, i.e., light sources which emit the VUV light over a continuum of wavelengths within the VUV range. A broadband source can be used for absorption spectroscopy in the VUV range. Because gases such as hydrogen and oxygen have resonance lines in this range, VUV absorption spectroscopy can be used for sensitive analytical measurements. A light source for use in spectroscopy desirably can operate continuously, with stable emission characteristics over time. A stable, continuously operable broadband VUV source is also needed for use as a calibration standard, for measuring the sensitivity of VUV light detection systems in laboratory apparatus.
Deuterium arc lamps have been used as VUV light sources. However, such lamps emit a relatively weak continuum or broadband radiation in the VUV range together with intense line radiation at particular wavelengths. This spectral characteristic requires that the detector system used to measure the light have a very high dynamic range, i.e., the capability of measuring a weak light at some wavelengths and very intense light at others.
Some consideration has been given to the use of excimer radiation as a source of UV light. Excimers are temporary chemical compounds composed of atoms that normally do not combine with one another. One or more of the atoms constituting an excimer is an excited state, i.e., a state in which the [electrons have] atom has been momentarily promoted to a higher energy state as, for example, by promoting one or more electrons to higher-energy orbitals. The excimer molecule as a whole is also in an excited state, and will ultimately decay to yield the constituent atoms. For example, elements commonly referred to as inert gases, helium, neon, argon, krypton and xenon, which normally exist only as isolated atoms can form excimer molecules when in the excited state. Diatomic inert gas excimers such as Ar2*, Kr2*, and Xe2* emit relatively broadband continuum radiation in the VUV range. However, to form these excimers in appreciable quantities, it is necessary to provide excitation energies on the order of 10 to 40 electron volts per atom. Moreover, this excitation energy must be provided while the gas is maintained at relatively low temperatures, typically below 200xc2x0 C. and most typically about room temperature. The gas also should be maintained under appreciable absolute pressure, desirably at least about 100 millibars (mbar) and most preferably about 0.5 bar or more, i.e., most preferably at about atmospheric pressure or more. Such substantial gas pressure is needed to provide a dense gas, which facilitates the excimer forming reactions. A simple direct current electrical arc discharge is ineffective to form excimers under these conditions. Other specialized arc discharge arrangements such as surface barrier discharges or arcs applied in short pulses can produce excimer light, but these devices operate only intermittently and do not provide stable, continuous emission.
Co-pending commonly assigned application Ser. No. 08/873,957, now U.S. Pat. No. 6,052,401, addresses the use of applying electron beams to gases to produce excimers to produce such broadband VUV light. However, all such electron beam approaches require the separate creation of an electron beam. It would be desirable to avoid the space and expense of producing an electron beam when creating the VUV light. Furthermore, the window for electron beam entry into the emission chamber still places limitations on the light source. Therefore, there is a need for broadband VUV light sources and monochromatic VUV light sources which can be produced at less expense and take up less space.
One aspect of the present invention provides methods of forming excimers in a gas. A method according to this aspect of the invention desirably includes the steps of providing free electrons in the gas disposed within a volume and imposing an electric field within such volume so as to accelerate the free electrons within the gas. The electric field is configured so that within a first region of said volume, said free electrons have mean energy equal to or greater than the excitation energy required for excimer formation. However, the field is configured so that within at least one region of the volume, the free electrons have mean energy less than the ionization energy of the gas. Stated another way, the field is configured so that any path through the electric field from negative potential to positive potential passes through at least one region of the volume in which the field is less than the field required to substantially ionize the gas. Thus, the free electrons excite the gas and form excimers without causing breakdown and arcing within the volume.
For example, the step of providing an electric field may include providing a point electrode within the volume and providing a counterelectrode remote from the point electrode, and imposing the electric field between the point electrode and the counterelectrode. As further discussed below, the field is very high in the immediate vicinity of the point electrode, but declines rapidly with distance from the point electrode. In this arrangement, free electrons may be provided in the gas by emission from the point electrode. The field immediately adjacent to the point electrode typically is so large that the mean energy of the electrons is far above the ionization potential of the gas, and the gas is substantially ionized and heated. The high temperature prevailing in this innermost region inhibits excimer formation in this region. In another region of the gas, immediately surrounding the innermost region, mean energy of the electrons is lower, and the temperature of the gas is lower. In this region, the field is such that the mean energy ranges from slightly above the ionization energy of the gas to below the ionization energy but above the excitation energy required for formation of the excimers. Substantial excimer formation occurs in this region. In yet another region, extending from the outside of the excimer-forming region to the counterelectrode, the field is below the excitation energy of the excimers and hence far below the ionization energy of the gas.
In another embodiment, an additional electrode may be provided. For example, the additional electrode and the counterelectrode may be provided as a pair of parallel plate electrodes. A substantially uniform field is maintained between the additional electrodes and the counterelectrode, with the counterelectrode being at a positive potential with respect to the additional electrode. This field has a substantially constant magnitude such that within this field, free electrons have mean energy equal to or greater than the excitation energy required to form the excimer but less than the ionization energy of the gas. Here again, a negative potential is applied to the point electrode and there is a high field in the immediate vicinity of the point electrode. There may be substantial ionization of the gas in the immediate vicinity of the point electrode. The point electrode and the surrounding ionized gas serve as a source of free electrons. Here, however, substantial excimer formation occurs within a region which extends from the outside of the innermost, hot region immediately surrounding the point electrode all the way to the counterelectrode.
The excimers decay and emit light, typically ultraviolet light, specifically in the vacuum ultraviolet region of the spectrum. Thus, a further aspect of the present invention provides methods of generating light such as VUV light. Typically, the gas is contained within a chamber, and the light is directed outside of the chamber through a window transparent to the light generated by the excimers.
Because the gas remains substantially non-ionized in at least some regions within the volume, there is no arcing or uncontrolled dielectric breakdown of the gas. The electrical discharge or electron flow from the point electrode has properties resembling those of a resistor. That is, the electron current increases with the applied voltage between the point electrode and the counterelectrode. The discharge is stable and can operate at steady state to provide a continuous source of excimers and continuous illumination. Moreover, multiple point electrodes can be operated in parallel to increase the amount of excimer formation and to increase the amount of light emission.
Preferred methods according to this aspect of the invention provide a new VUV light source which can operate at high gas pressures within the chamber or gas volume, because no window is needed for electron beam excitation. As high gas pressures contribute to excimer formation and discourage the formation of plasma conduits which lead to arc discharge, this allows a substantially brighter and smaller source of VUV light that has been previously available.
In further variants, multiple point electrodes connected in parallel can be replaced by an elongated electrode such as a wire or blade. The term xe2x80x9cfield emission electrodexe2x80x9d is used herein to denote an electrode which can emit electrons into the gas. Typically, such an electrode has a relatively small radius in one dimension (such as a blade or wire) or in two dimensions (such as the point of a needle) and provides a high local electric field in the vicinity of the electrode. Here again, a high field magnitude is provided in the vicinity of the elongated electrode. In still other embodiments, the source of free electrons does not include an electrode which provides a locally high field magnitude. For example, electron sources such as surface plasmas in a localized region of the gas, referred to as a xe2x80x9celectron source regionxe2x80x9d, can be employed. Such a plasma can be created by laser excitation. Electron beams can be employed, although the use of electron beams typically requires a beam window and hence limits the gas pressures which can be employed.
Another aspect of the present invention provides an apparatus for generating VUV light. Apparatus in accordance with this embodiment of the invention desirably includes, a chamber for holding a gas, means for providing free electrons in said gas; and means for imposing an electric field within a region of said gas so as to accelerate said free electrons, said electric field being configured as discussed above in connection with the methods.
In one exemplary apparatus, application of voltages of the order of magnitude of 10 kV to a metal needle tip electrode with a radius of curvature of approximately 20 xcexcm in an excimer-forming gas with a pressure on the order of 1-5 bars will produce VUV light. Although the present invention is not limited by any theory of operation, it is believed that the high negative voltage (on the order of 5 kV to 20 kV) at the needle tip of creates a high electric field around the metal needle tip electrode. This high intensity electric field is believed to generate free electrons through field emission from the metal needle tip electrode. The electric field accelerates these free electrons away from the metal needle tip electrode to an energy needed to cause excimer formation in a gas. These accelerated free electrons will collide with gas atoms or molecules and cause the excitation necessary to generate the VUV emitting excimers in the gas. This embodiment thus uses the phenomenon of point corona discharge (also known as St. Elmo""s Fire). It is important in harnessing the phenomena that no arc discharges occur.
These and other objects, features and advantages of the present invention will be more readily apparent from a detailed description of the preferred embodiments set forth below taken in conjunction with the accompanying drawings.